In the production of printed circuit boards, solid circuits are formed of a conductive material such as a foil positioned on opposite sides of a boardlike insulating base. In order that electrical connections may be established from the circuit made by the conductor on one side of the board to the conductor on the other side of the non-conductive board, it is common practice to form holes through the conductive sheets and insulating board and to conductively connect these sheets through these holes. These holes are called "through holes" or via holes and the connections may be by mechanical means such as rivets, eyelets or pins or by coating means such as electroplating a conductor on the surface of the via holes after first treating the non-conductive surface so that it may be electroplated.
The most widely used method of forming the connections is by electroplating copper. However, since the insulating board material is not receptive to electroplating, the inner surface of the via holes must first be treated or primed. A preferred method of doing this is by depositing a thin film of "conductivator" type metal such as palladium in colloidal form. A conductivator refers to a metal which is an activator and conductor at the same time, i.e. the metal has the combined functions of simultaneously acting as a conductor and catalyst or activator when placed on a dielectric base for subsequent electroplating while actually being a thin film composed of particles of colloidal or semi-colloidal size which film is substantially non-conductive. Metals which are considered conductivator type metals are metals which can be formed in liquid suspensions as colloids or semi-colloids, such as palladium or other metals capable of functioning in the same manner, i.e. copper, gold, silver, platinum, nickel, cobalt, and iron. An advantage in using palladium is its ability to strongly adhere to the dielectric or insulating board, as well as being very receptive to electroplating. Also, palladium is an auto-catalyst which means it will adhere to many different kinds of substrates.
This method of making electroplated connections in via holes in an insulator board, along with methods for putting metallized circuitry on the surface of the board, is fully described in U.S. Pat. No. 3,099,608, which issued July 30, 1963.
In the present art, integrated circuit chips are mounted on modules which involve either a metallized ceramic (MC) module or a metallized multilayer ceramic (MCL) module. The MC module is a single layer of metallurgy on the surface of a ceramic substrate with a limited extension to being made double sided because of the scarcity of wiring vias available. The MLC module has a layer of metallurgy on the surface of a ceramic substrate on which is superimposed dielectric, signal, and ground planes which are electrically interconnected by the plated via holes. These modules are provided with I/O pins which protrude from the bottom surface of the module and which plug into a printed circuit card or board to make electrical connection therewith. The I/O pins are mounted in the plated via holes in the module and the accepted practice is to bond them in the holes by means of solder. The solder connection may be made between the pins and the plated walls of the via holes or may take the form of solder globules which bond the ends of the I/O pins to the metallized surface of the module. If the solder is used between the pins and the walls of the via holes, the holes have to be made larger and this decreases the area available to receive the metallized circuitry and the mounting of circuit chips. If solder globules are used, they protrude onto the metallized surface of the module and this also decreases the area available for receiving metallized circuitry and the mounting of circuit chips. Also, the use of the protruding solder globules does not facilitate the manufacture of MLC modules because the superimposed dielectric and metallized layers are uneven which results in altering the electrical characteristics of the module.
Another disadvantage in using solder to fasten the I/O pins is that in the manufacture of these modules it is the accepted practice to use a low melt solder to bond the chip to the substrate, an intermediate melt solder to bond the ground planes, and a high melt solder to bond the I/O pins. When the I/O pins are plugged into a printed circuit card or board, they are soldered with a high melt solder. A problem arises when it is desired to make an engineering change wherein the module has to be removed. To remove the module, sufficient heat has to be used to melt the high melt solder to free the pins. This results in melting the low and intermediate solders and chips may fall out.
It became apparent that a method of providing conductive via holes and bonding of I/O pins was needed which did not involve the use of solder and wherein there would be no protrusions on the surface of the module which was to be metallized.
Attempts to provide such a method were tried by providing filled conductive via holes to which the ends of the I/O pins could be brazed. Filled conductive via holes have been proposed before. For example, one method involves the placing of silver balls over the via holes and melting the silver to fill the holes. However, it was found that this would not be feasible because the silver migrates and scatters causing short circuits. Also, it is highly corrosive. Another method is to fill the holes with a fine metallic powder. This does not result in a solid consistent filling and copper, which is preferred, does not adequately adhere to ceramic. Also, it has been proposed to use electrophoretic deposition of metal to fill the holes. However, this process does not use 100% pure metal and this introduces different electrical characteristics. Since copper is a preferred conductor, another method tried was to fill the previously described palladium plated via holes with melted copper. However, it was found that the temperature required to melt the copper, 900.degree. C-1200.degree. C, caused the palladium and the copper to diffuse into each other and there was not a sufficient amount of palladium left to adhere to the ceramic. It was also found that a copper filled via hole could not be satisfactorily produced by electroplating because the copper deposits in such a manner that a hole or void is left in the middle.
The problem presented, then, was to find a way of using the bonding characteristics of palladium and the conductive quality of copper to provide a filled via hole in a ceramic substrate.